The self-ionization of water is the process in which water ionizes to hydronium ions and hydroxide ions.
The self-ionization of water (the process in which water ionizes to hydronium ions and hydroxide ions) occurs to a very limited extent. When two molecules of water collide, there can be a transfer of a hydrogen ion from one molecule to the other. The products are a positively charged hydronium ion and a negatively charged hydroxide ion.
The equilibrium constant for the self-ionization of water is referred to as the ion-product for water and is given the symbol Kw and is derived from the equilibrium constant equation.
The ion-product of water (Kw) is the mathematical product of the concentration of hydrogen ions and hydroxide ions. At 25°C, the experimentally determined value of Kw in pure water is 1.0 × 10 -14 .
pH is used for indicating the concentration of H +. The letters pH stand for the power of the hydrogen ion. The pH of a solution is the negative logarithm of the hydrogen-ion concentration:
pH = -log[H+]
In pure water or a neutral solution, [H + ] = 1.0 × 10 -7 M. Substituting into the pH expression gives a value of pH 7.
Practice Questions
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Key Points
• The self-ionization of water is described and an ionization constant for the process is stated.
• Acidic and basic solutions are defined.
• Water can ionize to form a hydronium ion and hydroxide ion.
• Kw is the ionic product of water and has a value of 1×10-14.
• Using the negative logarithm of hydrogen ion concentration we can prove that pure water has a pH of 7.
• pH can be calculated by taking the -log of the H+ concentration
Key Terms
Acidic solution: A solution in which the concentration of hydrogen ions is greater than the concentration of hydroxide ions.
Basic solution: A solution in which the concentration of hydroxide ions is greater than the concentration of hydrogen ions.
Ion-product of water 
: The mathematical product of the concentration of hydrogen ions and hydroxide ions.
Self-ionization of water: The process in which water ionizes to hydronium ions and hydroxide ions.
pH: The negative logarithm of the hydrogen-ion concentration.